Once you reach the final Biome, there is an enemy called the Abyssal Trichozoid which plants itself on the ground and endlessly spawns the tentacle homing missile enemies called Anomos. Find an Abyssal Trichozoid with a red shield. As long as you don't have any perks that can destroy shields, stand in a safe spot and shoot the Anomos as they spawn until you unlock weapon perks and Proficiency.